This invention relates generally to magnetic resonance systems, and more particularly the invention relates to selective three dimensional excitation in magnetic resonance applications.
In magnetic resonance applications such as imaging and spectroscopy, a strong static magnetic field is employed to line up atoms whose nuclei have an odd number of protons and/or neutrons, that is, have spin angular momentum and a magnetic dipole moment. A second RF magnetic field, applied as a single pulse transverse to the first, is then used to pump energy into these nuclei, flipping them over, for example to 90.degree. or 180.degree.. After excitation the nuclei gradually return to alignment with the static field and give up the energy in the form of weak but detectable free induction decay (FID). These FID signals are used by a computer to produce images.
The excitation frequency, and the FID frequency, is defined by the Larmor relationship which states that the angular frequency, .omega..sub.o, of the precession of the nuclei is the product of the magnetic field, B.sub.o, and the so-called gyromagnetic ratio, .gamma., a fundamental physical constant for each nuclear species: EQU .omega..sub.o =B.sub.o .multidot..gamma.
Accordingly, by superimposing a linear gradient field, B.sub.z =z.multidot.G.sub.z, on the static uniform field, B.sub.o, which defines the Z axis, for example, nuclei in a selected X-Y plane can be excited by proper choice of the frequency spectrum of the transverse excitation field applied along the X or Y axis. Similarly, a gradient field can be applied in the X-Y plane during detection of the FID signals to spatially localize the FID signals in the plane. The angle of nuclear spin flip in response to an RF pulse excitation is proportional to the integral of the pulse over time.
Heretofore, selective RF and gradient waveforms have been used in spin echo imaging of two dimensional slices and in blood flow imaging by "tagging" or spin inverting of nuclear spins of blood flowing into a slice of non-inverted static material and then detecting the FID of the blood. In theory, multidimensional pulses should be designable to be selective in any number of dimensions. In practice, available gradient power has enforced a limit of two dimensions on excitations pulses.
The present invention is directed to three dimensional excitation pulses which are feasible on commercial imaging machines and to the magnetic resonance apparatus and methods utilizing the selective three-dimensional excitation pulses.